A bypass turbomachine has an upstream air inlet that is divided into a primary air stream feeding compression and combustion stages, and a secondary air stream bypassing the turbojet and reuniting with the primary air stream at the outlet from the turbojet.
A fraction of the air passing through the compressor is bled off for various purposes, including pressurizing the cabin, deicing, or ventilating the turbine of the turbojet in order to cool it.
Air is generally bled from the high-pressure compressor, which has an outer casing for stiffening the compression stage and a wall defining the outside of the flow passage for the primary air stream inside the high-pressure compressor and formed by an arrangement of annular segments, some of which carry stator vanes and others of which, alternating with the preceding segments, extend at the radially outer ends of rotor wheels.
The outer casing includes an orifice having a suction duct mounted thereon, which duct opens out into an annular space made between an annular segment of a nozzle stage and an annular segment of a rotor wheel.
In operation, a fraction of the air flowing along the primary air passage of the high-pressure compressor is bled from the inter-segment space and is conveyed to the equipment that requires air under pressure via the suction duct that opens out into the outer casing.
Nevertheless, air is bled off maximally in register with the end of the suction duct, which leads to a drop in static pressure at that location. Large heterogeneities of static pressure are thus observed in the inter-segment space around the axis of the turbomachine, thereby decreasing the performance of the turbomachine. Those heterogeneities with increasing requirements for bleeding off air.
In certain critical situations, such as an engine breakdown, the engine that remains in operation must be capable of delivering all of the bled-off air. For example, in normal operation, the maximum amount of air bled off represents about 8% of the mean flow through the compressor, and in the event of an engine breaking down, the other engine then needs to be capable of delivering up to 16% of its mean flow, which can be impossible if static pressure is highly non-uniform around the axis of the compressor.
Increasing the number of suction ducts around the outer casing is a solution that cannot be envisaged since that would complicate the pipework for delivering air under pressure and would increase the weight of the turbomachine.